The complexity of narrowband echo envelopes as a function of fish side-aspect angle

Abstract

High-frequency, narrowband acoustic signals may contain more information on.sh size and orientation than previously thought. Our observations of dual frequency identi.cation sonar (DIDSON) images of.sh orientation paired with split-beam echo envelopes helped clarify why metrics such as echo duration have performed better than target strength measurements when predicting salmon lengths at side aspect. Fish orientation has a pronounced effect on the duration and shape of split-beam echo envelopes from large (80-130 cm) salmon insoni.ed at side aspect. At near-normal aspect angles, echo envelopes are unimodal, symmetrical, and resemble echo envelopes from calibration spheres. With increasing oblique-aspect angle, echo shapes become less symmetrical as the number of peaks increases, and echo duration and amplitude become more variable. Using angle and range coordinates, peaks in an echo envelope can be traced to their origin on a DIDSON image. At oblique-aspect angles, discrete peaks develop that are re.ected from regions close to the head and tail. In addition, the distance between peaks increases with increasing aspect angle and is larger than can be explained by swimbladder length. © 2007 International Council for the Exploration of the Sea. Published by Oxford Journals.